The sinuous flow paths through the length of a fixed bed catalytic reactor provide for intimate contact between the reactant fluid and the active sites of the catalyst. The catalyst bed is also highly effective as a filter medium, retaining much of the solid material suspended in the reactant mixture. These suspended solids collect in the void spaces in the reactor bed causing increased pressure drop and non-uniform flow patterns.
One objective of fixed-bed reactor design is to maximize unit throughput while minimizing pressure drop across the reactor. Maintaining a uniform distribution of cross-sectional open area across the reactor bed promotes uniform flow and allows the maximum contact time between catalyst and reactants for a given fluid space velocity. Further, a uniform distribution of cross-sectional open area tends to discourage the accumulation of undesirable deposits in the catalyst bed.
The problem of catalyst bed plugging in downflow reactors is known to follow two distinct mechanisms. The first is plugging due to upstream corrosion products and other solids carried in the reactor charge stream. The second is plugging due to formation of solids within the catalyst bed.
For example, in hydrotreating reactors, plugs formed at the top of the catalyst bed (near the reactor inlet) typically consist of salts of iron and sodium. These salts are the products of corrosion reactions occurring upstream from the reactor. The current industrial practice is to install scale baskets (wire mesh baskets) at the top of the catalyst bed.
Plugs formed within the catalyst bed typically consist of metal sulfides resulting from the reaction of hydrogen sulfide with the organometallic compounds present in the feed.
The plugging problem is practicularly pronounced in reactors having catalyst loaded in longitudinal zones or beds. The plugging material in this case tends to migrate downward, filling the void space between the catalyst particles and collecting at the interface separating the two catalyst layers, thus accelerating the buildup of blockages and the increase in pressure drop.
Pressure drop continues to increase until continued operation is no longer safe and/or economicl. At this point, the reactor is taken out of service and cleaned. The cost of this industry-wide problem is reflected not only in lost profits due to process unit downtime, but also in the highly labor-intensive activity of mechanically removing catalyst and corrosion products from the reactor vessel.